/Engine/Source/Runtime/Experimental/Chaos/Public/Chaos/GJK.h
If you do roll your own function, make sure that the vector returned is in or on the Minkowski sum (and don't just use a unit vector along some direction for example) or GJKDistance may early-exit with an inaccurate result.
namespace Chaos { template<typename T, typename TGeometryA, typename TGeometryB> [TVector](API\Runtime\Chaos\Chaos\TVector)< T, 3 > Chaos::GJKDistanceInitialV ( const TGeometryA & A, const TGeometryB & B, const [TRigidTransform](API\Runtime\Chaos\Chaos\TRigidTransform)< T, 3 > & BToATM ) }